The blue ray discs are more expensive to produce, but they hold more data than the HD DVDs.  A special polymer hard coating called Durabis has to be applied to the blue ray discs in order to make certain they are hardy enough to protect all of the data against scratches and fingerprints that has been placed onto them and is only 0.1mm beneath its surface.  This was one of the major factors that helped drive the cost of production, and ultimately the cost of blue ray dics and products up.  The additional benefits though of having the data layer so close to the surface is that there is room for extra layers of data to be added to each disc.  The prices of blu ray are projected to come down as production gets mainstream.